1. Field of the Invention
The invention relates to a fuel supply apparatus for an internal combustion engine and a control method thereof, and more particularly relates to a fuel supply apparatus for an internal combustion engine including a fuel injection valve that injects fuel into the internal combustion engine, a delivery passage that delivers the fuel to the fuel injection valve, a fuel tank that stores the fuel, and a fuel pump that is driven by rotation of the internal combustion engine to regulate the fuel from the tank to a desired pressure and pump the fuel to the delivery passage, and to a control method thereof.
2. Description of Related Art
An apparatus including a fuel injection valve that injects fuel into an internal combustion engine, a common rail that holds the fuel to be supplied to the fuel injection valve in a high pressure condition, a fuel tank that stores the fuel, a fuel pump that pressurizes the fuel from the fuel tank using power from an output shaft of the internal combustion engine and supplies the pressurized fuel to the common rail, a pressure reducing valve that is opened and closed through electrification to allow the fuel in the common rail to flow out into the fuel tank via a low pressure fuel passage, and a fuel pressure sensor that detects a fuel pressure in the common rail, wherein the presence of an abnormality in the pressure reducing valve is diagnosed on the basis of a reduction in the fuel pressure in the common rail, detected by the fuel pressure sensor, when an ignition switch is switched OFF such that an opening operation is performed on the pressure reducing valve, has been proposed as this type of fuel supply apparatus for an internal combustion engine (see Japanese Patent Application Publication No. 2007-100624 (JP-A-2007-100624), for example). In this apparatus, the pressure reducing valve is determined to be normal when a reduction speed of the detected fuel pressure is no lower than a threshold at which the pressure reducing valve is assumed to be normal and determined to be abnormal when the reduction speed is lower than the threshold.
In the fuel supply apparatus for an internal combustion engine described above, however, it may be impossible to determine the presence of an abnormality in the pressure reducing valve properly when the fuel pressure in the common rail is low immediately before fuel injection through the fuel injection valve is stopped. For example, in a vehicle installed with an internal combustion engine as a power source, the ignition switch may be switched OFF such that an opening operation is performed on the pressure reducing valve while the internal combustion engine is operating at a low load or no load, and in this case, the fuel pressure in the common rail immediately before the opening operation on the pressure reducing valve is in a reduced state required for the internal combustion engine to operate at a low load or no load. Hence, even when the pressure reducing valve is opened in this state, it may be impossible to obtain a fuel pressure reduction speed required for the diagnosis, and as a result, it may be impossible to diagnose the pressure reducing valve properly. Further, when a pressure reducing valve that requires electrification for opening/closing control is used, an abnormality outside of a pressure reducing valve main body, such as an abnormality in a drive circuit for driving the pressure reducing valve, may occur, and it is therefore desirable to be able to reduce the fuel pressure in the common rail using a simpler structure.